I used to have to reduce whole bottles of port down to a syrup, and found that slower was better, if only because 1) the alcohol fumes were less likely to catch fire (which they did when I used high heat), and 2) it didn't reduce down to a pot full of carbon while I wasn't paying attention

which could happen much more easily when it was boiling hard.
For reducing vinegar, again, I'd be gentle so as not to release fumes too violently into the air. Your mates will appreciate that.